Vacuum metalizing of plastic and similar dielectric substrates has been practiced for some time. The automobile industry has used metalized trim components that could be substituted for conventional chrome-plated metal parts. The stages of metal film growth by vacuum deposition proceeds from metal nucleation and nuclei growth, liquid coalescence, to electrically discrete islands, channelization with incipient film conductivity and finally full continuous film formation. U.S. Pat. No. 4,431,711, issued Feb. 14, 1984 in the name of Eisfeller, assigned to the assignee of the subject application, discloses one method of vacuum metalizing a dielectric substrate with indium and products thereof. The patent relates to an article of manufacture comprising an organic dielectric base or substrate having a smooth surface such as a molded plastic. A macroscopically continuous-appearing very thin layer thereon of a vacuum deposited corrosion-prone metal, specifically indium and alloys thereof is deposited on the smooth surface. The metal is in the form of minute specular electrically discrete rounded metal islands. There is a top coating over the metal film of an intimately adhered clear dielectric resin film encapsulating and protecting the metal particles, and binding them firmly to the substrate. The product is particularly useful in automotive applications such as an automobile exterior trim component to replace heavier and more expensive conventional chrome-plated metal parts.
During the vacuum metalizing process, a vacuum chamber encompasses the dielectric substrate to be coated along with evaporator boats having metal material thereon, which material is vaporized and attracted to the substrate. Due to the unique operating characteristics of the vacuum metalizing process, it has been determined that matched evaporation boats, i.e., having similar evaporation rate or heating rates, provide for a better part quality and a better first run capability. In general, a plurality of evaporation boats are connected in parallel or series to the power source during evaporation of the metal material. It is important to that evaporation boats have similar evaporation characteristics to ensure an even, uniform layer of metal on the substrate during the evaporation process. When unmatched boats are utilized, some of the boats have evaporation rates different from other boats and therefore an uneven coating may occur on the substrate due to the different rates of metal evaporation. Because of the unique system in which the tungsten boats are used, there is no method to determine which boat is providing more or less evaporated metal material to the vacuum metalizing process.